1. Field of the Invention
This invention relates to a process for producing aromatic urethanes. More particularly, it relates to a process for producing aromatic urethanes by reacting aromatic nitro compounds, organic compounds containing at least one hydroxyl group (hereinlater referred to as hydroxyl group-containing organic compound) and carbon monoxide under high temperature and high pressure conditions in the presence of a catalyst of a specific type.
2. Description of the Prior Art
As well known, urethanes have been heretofore produced mainly by reaction of isocyanates and hydroxyl group-containing organic compounds. In recent years, several novel processes for producing urethanes have been proposed because of the lack and high cost of starting materials for producing the isocyanates and also high toxicity of the intermediates derived from the starting materials. However, such newly developed processes have several vital drawbacks and have not been yet put into practice on an industrial scale.
For instance, there has been proposed a process in which an aromatic urethane is produced from an alcohol, carbon monoxide and an aromatic nitro compound by use of a rhodium chlorocarbonyl catalyst (U.S. Pat. No. 3,338,956). However, this process is not economically advantageous in producing highly pure aromatic urethanes since the yield of the urethane product is low even if the reaction is effected in the presence of a large amount of a catalyst for a long period of time.
In order to improve the above process, there has been also proposed another process using a compound containing a carbonyl group of a metal of the group VIII of the Period Table and a metal salt capable of existing in a state of a di- or higher valence such as ferric chloride (German Pat. No. 1,543,051). However, this process is not practical since the yield of a urethane product is still low even when a mononitro compound is used as a starting material and use of a dinitro compound will result in lower yield.
There is also known a process using palladium and a Lewis acid as a catalyst (U.S. Pat. No. 3,531,512). According to the process, even when a dinitro compound is employed as a starting material, the yield of urethane as high as 80-90% may be attained. In order to attain such high yield, however, the reaction must be conducted under severe conditions such as of an initial pressure of carbon monoxide of 190-350 kg/cm.sup.2 and a reaction temperature of 190-200.degree. C. In addition, the process involves an industrially serious problem that a Lewis acid, e.g., ferric chloride, serving effectively as a promoter exerts a considerable corrosive action on a metal material such as iron, stainless steel, or the like. In order to realize the process on an industrial scale, it is accordingly essential to use a glass or tantalum reactor, offering serious economical and technical problems.
There is known a further process using a catalyst composed of a platinum group metal compound and a tertiary amine (U.S. Pat. No. 3,993,685). However, this process needs a large amount of the catalyst and is thus uneconomical.
Quite recently, there has been proposed a process in which a ternary catalytic system composed of a member selected from palladium, ruthenium, rhodium and a compound thereof, a Lewis acid, and a tertiary amine is used and the reaction is conducted in coexistence with water, if desired (Japanese Patent Application No. 69721/'76). However, even this process can hardly produce aromatic urethanes of high quality and excellent heat stability.